The use of articulating block matrices for soil erosion prevention is known in the art. Typically, such systems involve the grading of an embankment or shoreline to a predetermined slope, the installation of a highly water permeable geosynthetic fabric over the soil substrate, and then the placement over the fabric of a matrix of blocks. A typical matrix of blocks is comprised of precast concrete blocks, tied together into mats with cables usually comprised of high strength polyester or galvanized steel. These mats are typically assembled off-site at a block precasting facility. After the blocks are cast, cables are strung through ducts in the blocks, typically producing mats that are approximately 8 feet wide and 40 feet long. Mats of this size have proven convenient for handling and transporting to the job site. The assembled mats are lifted onto a truck or barge for transportation to the job site using a crane or large forklift truck equipped with a spreader bar assembly which suspends the mats in a generally horizontal orientation. At the installation site, the mats are placed side by side by a crane using a spreader bar assembly. The cables of adjacent mats are bonded together so that the finished installation comprises a continuous matrix of concrete blocks. The openings in the resulting surface may be backfilled with soil and seeded to produce vegetation. The presence of vegetation produces an aesthetically appealing shoreline and also provides greater resistance to erosion.
A revetment system constructed in this manner relies on the combination of the permeable fabric and the articulating concrete block surface to overcome the erosive effects of flowing water or waves to hold in place the underlying soil. Such systems have been widely used, and there are numerous examples of revetment systems that operate in the general fashion described above, including those described in. U.S. Pat. No. 4,227,829 (Landry), U.S. Pat. No. 4,370,075 (Scales) and a system marketed by Revetment Systems, Inc. under the tradename PETRAFLEX.TM. Revetment System.
The revetment system described in the Landry patent shows a matrix of blocks arranged such that the blocks are arrayed in parallel transverse rows and parallel longitudinal columns. The blocks are not shaped to interlock with each other in the matrix but are connected together with sets of cables passing through tunnels in the blocks. One set of cables passes through the entire transverse dimension of the matrix and another set passes through the entire longitudinal dimension of the matrix. For convenience, this system is referred to as a "dual cable system" below. The blocks have angular tapered sides such that the top surface of the block has less surface area than the bottom surface, to facilitate articulation of the matrix over non-planar surfaces and bowing of the matrix when it is suspended from a spreader bar assembly.
Like Landry, the revetment system described in Scales is a matrix of blocks placed in parallel transverse rows, with cable interconnections. The blocks also have angular tapered sides to facilitate articulation. Unlike Landry, the revetment system described in Scales uses cables that travel only in the longitudinal direction and each block has two longitudinal tunnels for the cables. For convenience, this system is referred to as a "single cable system" below. The blocks of Scales are of a generally rectangular shape, with recesses and protrusions in the sidewalls configured so that longitudinally adjacent blocks interlock when the blocks are placed in a "running bond" pattern in the matrix by off-setting adjacent transverse rows in the transverse direction.
In the PETRAFLEX.TM. System the blocks are, like the blocks of Landry, generally square, and are placed in parallel columns and rows with a dual cable system. Two tunnels, each accepting one cable, are used in the longitudinal direction, and one tunnel, accepting one cable, is oriented in the transverse direction. Unlike Landry, the block of the PETRAFLEX.TM. system has, for each pair of sidewalls, one male tab on a side opposed to one female tab on the other side to interlock adjacent blocks when placed in a matrix with parallel rows and columns of like blocks.
The manner in which the blocks are placed into a matrix is an important design feature of articulating block revetment systems. The prior art teaches the use of cables connecting the blocks and providing a block to block interlock by shaping the blocks so that they nest together when placed in a matrix. The prior art also includes blocks that are laid without using interconnecting cables and which rely on the unit mass and block to block interlock to maintain the blocks in place. The dual cable systems perform well, but require additional cable over that required by the single cable systems, and are more costly as a result. The single cable systems and systems not using any cables do not perform as well as the dual cable systems, but may be more cost-effective for certain applications. While the use of cables is desirable for system strength and to prevent removal by vandals, blocks without cables can be hand-placed, which is an advantage in certain applications. For all systems, performance is improved by increasing the amount of block to block interlock to restrict lateral movement of adjacent blocks.
Each of the Scales, Landry and PETRAFLEX.TM. system block designs are designed to be placed into a matrix in only one way and with only one cabling system. There is a need for an easily manufactured block that can be constructed into multiple matrix configurations with a high degree of interlock between adjacent blocks to suit the design requirements of varying site conditions, so that the blocks can be configured to function with a multiple cable system, a single cable system, or no cables at all.
Another important design consideration for revetment systems is their ability to allow water to flow through the surface of the concrete mats. In most settings where such systems are used, water may be present in the soil substrate underneath the layer of geotextile and the concrete block mat. Such water may be introduced through rainfall, surface flows, wave action, subsurface groundwater flows or other elements. As a result, it is highly desirable that the surface of the block matrix be permeable so that the matrix is not displaced by hydrostatic pressure or undermined by erosion caused by flows occurring in the soil substrate beneath the block matrix and geotextile. It is common practice to have open voids in the matrix consist of approximately twenty percent (20%) of the total surface of the block matrix. Such voids are located either within the blocks or in the spaces between the blocks when they are placed in the matrix. There are also instances, however, where a unit without such open voids may be desired.
While such openings are highly desirable, they do introduce an element of vulnerability to displacement of the blocks, because such voids may allow wave action or water flows to destabilize or undermine the matrix. Thus, there is a need for an improved design for such voids that minimizes the disruptive effect of hydrodynamic forces while providing sufficient open area to allow the release of water that may accumulate beneath the surface of the matrix. Such a design ideally should be able to address the fact that hydrodynamic forces may act on the revetment structure from different directions in different applications. For example, when the revetment is intended to protect a river embankment, the forces typically will come from the flow of water along the transverse direction of the matrices. When the revetment is installed to protect a dam overtopping, then the forces are oriented along the longitudinal direction. When the revetment is installed along a shoreline, the forces may be along the longitudinal direction, or diagonal to it. Thus, the orientation of the forces is an important factor in addressing the hydraulic efficiency of the revetment matrix. To adequately address this issue could require a multitude of different block designs, but such a multitude does not allow economies of scale in production, and complicates and increases the expense of the manufacturing process. There is a need for one multipurpose block that is capable of being placed in multiple orientations in a prefabricated matrix such that it can have maximum hydraulic efficiency for the particular, and potentially conflicting, requirements of different jobs.
Another important factor regarding the configuration of the block openings is the dispersion of the openings across the top surface of the block to allow greater coverage and concealment of the block structure by vegetation that is planted in the openings. There is a need for a block that spreads the openings in the block widely across the top surface to allow greater vegetation coverage.
Another important characteristic of a revetment system is the shear resistance of the block, geofabric and soil interface. The blocks used in the prior art revetment systems have planar bottom surfaces and rely simply on the weight of the blocks and friction to overcome shear forces at this interface. In some instances, this has resulted in local system failures. The performance of the system could be significantly enhanced by improving the shear resistance of the geofabric and block matrix against lateral displacement along the soil interface. There is a need for a block design which has a cost-effective gripping configuration built into its bottom surface to improve the shear strength at this lower interface.
Finally, there is a need for more cost-effective means of protecting the cables that tie the blocks together against abrasion. During the process of handling the mats after they are assembled, the mats are lifted using the cables. This results in weakening of the cables at the mouths of the tunnels of the blocks where the cables rub against the rough surface of the concrete. This problem has been addressed in the prior art by providing for an insert that is passed through the entire length of the matrix (See U.S. Pat. No. 4,227,829 to Landry). While such approaches serve their intended purpose, there is a need for a less expensive means of accomplishing the task of protecting the cables against abrasion at critical locations.